Fluid Management Part 1

Fluid compartments: 

The body is made of about 60% water, which is divided between the intracellular and extracellular space. Fluid is distributed in these spaces by osmotic forces determined by ion distribution between the intracellular and extraceullular space. Approximately 2/3rds (40% total body weight) of the body’s fluids is distributed in the intracellular space and 1/3rd (20%)in the extracellular space.

The extracellular space is further divided into the vascular space and the interstitial fluid. The intravascular space also has an intracellular component made up of red and white blood cells. It makes up approximately 10% of the body’s fluids (6% total body weight). Fluid is distributed between the vascular space and the interstitial space through starling forces . Starling forces describes a combination of hydrostatic forces and oncotic pressure from protein molecules between capillaries and the interstitial space. At the arterial end of the capillary, high hydrostatic forces in the capillary compared to the interstitial space causes fluids to flow into the interstitial space. At the venous end of the capillary high oncotic forces in the capillary compared to the interstitial space causes the fluid to move back into the vascular space. 

In addition to this some fluid in the body is secreted out into the bowel – this is known as the third space. 

Types of IV fluids
IV fluids can be divided into water, crystalloids and colloids.

Different types of IV fluids are distributed throughout the fluid compartments differently, which is relevant in resuscitation situations where the volume of the intravascular space needs to be increased to improve blood pressure and perfusion to organs.

Fluids containing only glucose or dextrose is metabolized leaving only water which is distributed across the intracellular, interstitial and vascular spaces. Because of this only about 10% of the volume of dextrose administered remains in the vascular space. 

Crystalloids refer to fluids containing ions such as 0.9% saline solution or Hartmann’s solution. Here the fluid is retained in the extravascular space by the osmotic forces exerted by sodium, and distributed between the interstitial and vascular space only. About 30% of the administered volume therefore remains in the vascular space.

Colloids refer to fluids containing large molecules which may be synthetic or natural such as gelafusin or albumin. Colloids retain fluids in the vascular space as the oncotic pressure exerted by the molecules draw water into the vascular space. Theoretically 100% of the volume of colloids administered remains in the vascular space.

Daily losses and needs:

A 70kg man will typically loose about 2500 mls of water per day

The kidney has to secrete a minimum amout of urine to excrete waste. This is known as the minimal obligatory volume of urine and is usually between 0.5-1ml/hr/kg. In addition fluids are lost from faeces, sweat, and moisture from the lungs. 

Daily losses:

• Urine (1500mls)

Insensible losses:

• Faeces (100mls)
• Sweat and moisture from lungs (900mls)

Daily losses are usually replaced through fluids in food and drink in health.

Fluid input:

• Oral fluids (1200mls)
• Food (1000mls)
• metabolism (300mls)

Fluid replacement may be needed if a patient is unable to replace losses or due to increased losses. Some sources of increased fluid loss include:

• Vomiting/Diarrhoea
• Stomas
• Mechanical ileus
• Fever
• Bleeding
• Burns
• Polyuria

Additional resources:

1.  Lobo DN, Lewington AJP, Allison SP, Basic concepts of fluid and electrolyte therapy, Melsungen, Die Deutsche Bibliothek, 2013 [online book]
2. National Institute for Health and Care Excellence (NICE), CG174, Intravenous fluid therapy in adults in hospital, London,  NICE, 2013

References:

1. Goldberg A, Stansby G, Surgical Talk Surgery for Finals, London, Imperial College Press, 2004.
2. National Institute for Health and Care Excellence (NICE), CG174, Intravenous fluid therapy in adults in hospital, London,  NICE, 2013
3. Hadjipavlou M, Chew GWM, Farmery JS, Fluid management, Student BMJ, 2010:18:c5063